1. Field of the Invention
The present invention relates to the use of austenitic stainless steels in applications requiring anti-coking properties.
In accordance with the invention, these steels are used to manufacture equipment, for example furnaces, reactors or ducts, or elements for producing such equipment, or to coat the internal walls of furnaces, reactors or ducts, this equipment being used in particular to implement petrochemical processes carried out at temperatures of 350xc2x0 C. to 1100xc2x0 C. and during which coke can be formed.
The invention also relates to reactors, furnaces, ducts or their elements produced from or coated with these steels.
The carbonaceous deposit that can develop in furnaces during hydrocarbon conversion is usually known as coke. This coke deposit is a nuisance in industrial units. The formation of coke on the walls of tubes and reactors causes a reduction in heat exchange, major blockages and thus, an increase in pressure drops. To keep the reaction temperature constant, the temperature of the walls may have to be increased, risking damage to the constituent alloy of the walls. A reduction in unit selectivity is also observed, resulting in a reduction in yield.
2. Description of the Prior Art
Japanese application JP-A-03-104 843 describes refractory anti-coking steel for an ethylene cracking furnace tube. However, that steel contains more than 15% of chromium and nickel. It was developed to limit coke formation between 750xc2x0 C. and 900xc2x0 C. for ethylene cracking.
U.S. Pat. No. 5,693,155 relates to petrochemical processes using stainless steels rendered less coking by adding up to 5% of silicon. Such steels contain at least 10% of nickel, which makes them expensive.
Further, French patent application FR-A-2 766 843 describes an austenitic stainless steel with a low nickel content, which is cheaper compared with the standard grade (AISI 304), but which has equivalent mechanical and welding properties.
That steel has the following composition:
0.1% to 1% of silicon;
5% to 9% of manganese;
0.1% to 2% of nickel;
13% to 19% of chromium;
1% to 4% of copper;
0. 1% to 0.40% of nitrogen;
5xc3x9710xe2x88x924% to 50xc3x9710xe2x88x924% of boron;
at most 0.05% of phosphorus; and
at most 0.01% of sulphur.
We have now discovered that steels of the above type have good anti-coking properties and can advantageously be used for the manufacture of equipment, for example furnaces, reactors or ducts, or elements of equipment, for example tubes, plates, sheets, screens, profiles or rings, or to coat the internal walls of furnaces, reactors or ducts, said equipment being intended to implement petrochemical processes carried out at temperatures of 350xc2x0 C. to 1100xc2x0 C. and in which coke can be formed.
The present invention concerns the use of stainless steels with a composition specific for the production of good coking resistance, but which retain an austenitic structure despite a reduced nickel content. The high temperature behaviour of stainless steels with an austenitic structure combines good corrosion resistance with good mechanical behaviour, including weldability.